A fundamental part of the life cycle of higher plants is the alternation between a diploid, sporophytic generation and a haploid, gametophytic generation. In flowering plants, the gametophytic generation consists of pollen grains and the embryo sac within the ovary. The transition from the sporophytic phase to the gametophytic phase in higher plants consists of two processes, sporogenesis and gametogenesis. Gametogenesis mainly involves the differentiation of haploid spores into mature gametophytes. See G. N. Drews, et al., Plant Cell 10(5) (1988). Sporogenesis is characterized by the differentiation of hypodermal cells in anthers and ovule primordia into archesporial cells that further develop into microsporocytes (pollen mother cells) and megasporocytes (egg mother cells). See J. Bowman, (1994) Arabidopsis, An Atlas of Morphology and Development. The microsporocytes and megasporocytes (collectively known as meiocytes) undergo meiosis to produce spores. The formation of meiocytes thus comprises a very important step in plant reproduction.
In Arabidopsis, sporogenesis and gametogenesis (also known as megasporogenesis and megagametogenesis, respectively) have been well described. See Bowman, J., 1994, Arabidopsis, An Atlas of Morphology and Development. In sporogenesis, bitegmic and tenuinucellate ovules arise as finger-like structures on the placenta in the ovary (carpel) of the plant. A single hypodermal cell at the top of the ovule primordia becomes more prominent than neighboring cells and more conspicuous nucleus, and differentiates into an archesporial cell in stage 10-11 flowers. The archesporial cell then elongates and polarizes its cellular components longitudinally and differentiates into a sporocyte or megaspore mother cell (MMC). The MMC then undergoes meiosis to form four haploid megaspores (tetrad). Shortly after the archesporial cell becomes visible, in stage 11 flowers, the inner and outer integuments form from epidermal cells at the base of the nucellus. In gametogenesis, the outer integument overgrows the inner integument and both inner and outer integuments envelop the nucellus in which the female gametophyte (embryo sac) develops during stage 13. At mature stage, the inner cell layer of the inner integument differentiates into a nutritive endothelium (integumentary tapetum).
Although the above is well known, little is known about the molecular and genetic mechanisms that regulate and control sporogenesis, especially meiocyte formation. The identification of genes that regulate and control meiocyte formation could help understand these mechanisms and find ways to manipulate the fertility of plants.
An object of the present invention thus is to provide isolated nucleic acids and encoded proteins involved in meiocyte formation in plants, which can be used to manipulate plant fertility.
Another object of the present invention is to produce plants in which meiocyte formation has been affected during growth to render the plant capable of bearing altered fruits and/or altered flowers, including seedless fruits and pollenless flowers.